This invention relates to a digital type combustion control apparatus and more particularly to an improvement in the time for controlling the start of reignition operation when fuel flaming is extinguished during combustion in such an apparatus.
In a combustion control apparatus for a combustion system which generates a hot blast or heated water by burning gas fuel or liquid fuel such as kerosene, before combustion operation is usually performed a prepurge operation, i.e. operation to remove the residual fuel in the combustion system, by driving a blower. Then the fuel is supplied by actuating the fuel supply valve to open and the igniting operation is continued for a certain time by energizing the ignitor so that the supplied fuel may be burned. In this case, if fuel is not burned for a predetermined time after the start of the ignition operation, a warning apparatus operates to indicate that a fault has taken place and to stop the entire operation of the combustion system. In case where fuel flaming is extinguished for some reason or other while fuel has been burning so far, after the completion of igniting operation, the reignition operation is performed after the combustion control cycle has been started with the prepurge operation (prepurge return method) or the reignition operation starts without the prepurge operation (ignition return method). The warning apparatus is operated, irrespective of the kind of methods, unless fuel flaming is resumed in a predetermined time, i.e. safety time, after the start of reigniting operation. This invention is applicable to both of the methods. Also, the term "reignition operation" includes the same action in both of the methods. In the conventional combustion control apparatus of digital type, the timer for measuring the safety time usually comprises flip-flops for counting reference clock pulses. Since the reference clock pulses are not synchronized with a signal indicating that fuel burning is extinguished (hereafter referred to for convenience as flame extinction signal), then an error equal to one period of the clock signal at most is incurred in the safety time. In the case where the safety time is selected to be relatively short a relatively large error will possibly be caused. For example, a combustion apparatus with gas fuel according to the direct ignition method or ignition return method has a safety time of 2 seconds and if in this case the period of the reference clock signal is one second, the error in the sense mentioned above becomes considerably large. Accordingly, the effective portion of the safety time is shortened, the probability of ignition during the safety time decreases and the chance of failure in ignition increases.
Even in the case where a long safety time is used, the same problem will be caused if the period of the reference clock pulse signal is lengthened.
Moreover, in the prepurge method where prepurge operation is performed prior to the ignition operation after fuel burning is extinguished, there is caused a problem in that the time of prepurge operation or prepurge time is shortened.